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<?php
// This is a newer, better method
// First, grab the current page number from the URL
// If using a static front page, use 'page' instead of 'paged'
$the_page = get_query_var('paged');

// Standard custom query, ensuring to add the 'paged' parameter
$args = [
  'post_type' => 'testing',
  'posts_per_page' => 3,
  'paged' => $the_page
];

$query = new WP_Query($args);

while ( $query->have_posts() ) : $query->the_post();
  // Loop stuff
endwhile;

// Set up pagination
// Since this is a custom query, on a custom page template, we need to manually tell the function how many total pages can be paginated
echo paginate_links([
  'total' => $query->max_num_pages
]);

<?php
// An older method, a hack really that works by reseting the default WP_Query in order to trick WordPress into thinking that our custom query is the default query
// Not a fan of this method, but post it because it is still a reliable alternative

// Store the orginal, default WP_Query in a temp variable
$temp = $wp_query;

// Flush the default WP_Qeury
$wp_query = null;

// Set up our custom query, making sure to name it the same as the global $wp_query that WordPress expects
// Note that because we are hijacking the orginal $wp_query, we dont need to manually set the $paged variable
$args = [
  'post_type' => 'testing',
  'posts_per_page' => 3,
  'paged' => $paged
];

$wp_query = new WP_Query($args);

while ( $wp_query->have_posts() ) : $wp_query->the_post();
  // Loop stuff
endwhile;

// We can now use any pagination function that works out of the box
the_posts_pagination();

// Be sure to flush our hacked $wp_query
$wp_query = null;

// And restore it to its orginal self for WordPress to work with
$wp_query = $temp;